This invention relates to a mass flowmeter designed to operate by the Coriolis principle, incorporating an essentially straight fluid-conducting Coriolis measuring tube, at least one oscillator associated with and exciting the Coriolis measuring tube, at least one detector associated with the Coriolis measuring tube and capturing the Coriolis forces and/or the Coriolis oscillations generated by Coriolis forces, and a compensating cylinder in which the Coriolis measuring tube is mounted by way of a mechanical connection between the Coriolis measuring tube and the compensating cylinder.
The above states that the mass flowmeter in question incorporates at least one oscillator xe2x80x9cassociatedxe2x80x9d with the Coriolis measuring tube, and at least one detector xe2x80x9cassociatedxe2x80x9d with the Coriolis measuring tube. Typically, the oscillator or oscillators, or at least part of the oscillator(s), and the detector or detectors or at least part of the detector(s), are connected to the Coriolis measuring tube. However, since such connection is not a must, the term used herein is xe2x80x9cassociatedxe2x80x9d rather than xe2x80x9cconnectedxe2x80x9d.
There are two basic types of mass flowmeters operating by the Coriolis principle, one employing a more or less straight Coriolis measuring tube, the other a looped Coriolis measuring tube. As another differentiating feature, there are mass flowmeters with only one Coriolis measuring tube and those with two Coriolis measuring tubes, in the latter case permitting either parallel or in-line flow of the fluid.
In recent times, mass flowmeters employing only one essentially straight Coriolis measuring tube have increasingly gained in popularity. Compared to mass flowmeters using either two straight Coriolis measuring tubes or one looped Coriolis measuring tube, Coriolis-type mass flowmeters with only one straight measuring tube offer significant advantages. The advantage over mass flowmeters with two straight Coriolis measuring tubes lies primarily in the fact that, in contrast to dual Coriolis measuring tubes, single-tube designs do not require a flow divider or flow combiner. Compared to single or dual looped Coriolis measuring tubes, the main advantage of the straight Coriolis tube design is that it is easier to manufacture than a looped Coriolis measuring tube, that there is less of a pressure drop in a straight Coriolis measuring tube than in a looped Coriolis measuring tube, and that a straight Coriolis measuring tube is easier to clean than a looped Coriolis measuring tube.
Still, all these advantages notwithstanding, mass flowmeters with only one straight Coriolis measuring tube present problems in a variety of ways.
First of all, in a straight Coriolis measuring tube, thermal expansion and stress cause variations in the measuring accuracy as a function of the temperature of the moving fluid. In extreme cases, thermal stress can even lead to mechanical defects such as stress-induced fissures in the Coriolis measuring tube.
The above-mentioned problems with mass flowmeters employing straight Coriolis measuring tubes have already been addressed by the industry (reference is made in particular to German patent 41 24 295, German patent disclosure 41 43 361 and the German patent 42 24 379). The problems have been largely solved, on the one hand, by connecting the Coriolis measuring tube with the compensating cylinder in such fashion that any relative movement in the axial direction is inhibited whereby the axial distance of the connecting point between the Coriolis measuring tube and the compensating cylinder defines the length of oscillation of the Coriolis measuring tube, and, on the other hand, by positioning the Coriolis measuring tube in the compensating cylinder in tensile-prestressed condition (German patent 41 24 295), and/or by producing the Coriolis measuring tube and the compensating cylinder from materials having identical or nearly identical coefficients of thermal expansion (German patent disclosure 41 43 361), and/or by providing a length-variation sensor capable of detecting changes in the oscillation length of the Coriolis measuring tube and of correcting the measurements for varying oscillation-length and stress factors (German patent 42 24 379). Most notably, it has been possible to produce a Coriolis-type mass flowmeter employing a single Coriolis measuring tube with a measuring accuracy of within about 0.1% (ref. Prospectus for the xe2x80x9cZulassung des Corimass G-Gerates zum elchpflichtigen Verkehrxe2x80x9d issued by KROHNE Messtechnik GmbH and Co. KG).
However, mass flowmeters operating by the Coriolis principle and employing a straight Coriolis measuring tube do have one inherent drawback (ref. European patent disclosure 0 521 439):
It is necessary for the Coriolis measuring tube or tubes in mass flowmeters operating by the Coriolis principle to oscillate under the action of at least one oscillator; after all, it is the oscillation of the Coriolis measuring tube or tubes and the flow of mass through the Coriolis measuring tube or tubes that produces the Coriolis forces or Coriolis oscillations.
In mass flowmeters employing two straight Coriolis measuring tubes or one or two looped Coriolis measuring tube(s), the Coriolis measuring tubes or the active oscillating sections of the looped Coriolis measuring tubes are identical in design and so positioned and excited, that they oscillate in mutually opposite directions. As a desirable result, the overall oscillating structure has no external oscillating effect. The center of inertia remains stationary, compensating for any forces encountered. It follows that no oscillations are introduced into a pipeline system in which this type of mass flowmeter is installed, so that no pipeline vibrations will affect the accuracy of the measurements.
Of course, Coriolis-type mass flowmeters employing only one straight Coriolis measuring tube do not offer the benefit of counter-oscillating measuring tubes. The center of mass does not remain stationary and there is no compensation for spurious forces. As a result, a mass flowmeter of this type when installed in a pipeline will transfer vibrations into the pipe which, in turn, can affect the measuring accuracy. The industry has already addressed the task of minimizing the introduction of extraneous interferences, i.e. vibrations in the surrounding pipeline structure (ref. German patent disclosures 44 23 168 and 196 32 500).
To control the aforementioned problems which are peculiar to Coriolis-type mass flowmeters employing only one straight Coriolis measuring tube, the pipeline system in which the mass flowmeter is installed is often provided with additional clamping devices. As a rule, the pipe through which the fluid flows to the mass flowmeter and the pipe through which the fluid is carried away from the mass flowmeter are clamped down at spatial intervals corresponding to ten to fifteen times the pipe diameter.
Another proposed approach to the aforementioned problems which are peculiar to mass flowmeters operating by the Coriolis principle and employing only one straight Coriolis measuring tube, has been to install so-called antiresonators at the point where the Coriolis measuring tube is mounted, which antiresonators should have a resonant spectrum of a bandwidth that matches at least one intrinsic, natural vibration of the Coriolis measuring tube (ref. European patent disclosure 0 521 439). It has been found, however, that in the case of mass flowmeters which are very accurate to begin with, this approach offers no further improvement in terms of measuring accuracy or error reduction.
Another approach has been, especially in the case of a mass flowmeter employing only one straight Coriolis measuring tube, to mount on the compensating cylinder an equalizing unit of a symmetrical design and positioned in a symmetrical relation to the center of the Coriolis measuring tube (German patent disclosure 197 10 806). That equalizing unit must be so designed that the oscillation amplitude of the compensating cylinder is minimal and preferably close to zero.
Finally, it has recently been proposed that for a mass flowmeter with only one straight Coriolis measuring tube both the excitation oscillation and the Coriolis oscillation of the Coriolis measuring tube within the compensating cylinder be balanced (ref. German patent disclosure 198 40 782). The term xe2x80x9cbalancedxe2x80x9d in this case refers to a situation where the compensating cylinder is affected neither by the excitation oscillation nor by the Coriolis oscillation. In other words, neither the excitation oscillation nor the Coriolis oscillation will induce a xe2x80x9ccompensating-cylinder oscillationxe2x80x9d; the compensating cylinder remains unaffected, or xe2x80x9cin a state of equilibriumxe2x80x9d. That invention thus recognizes the fact that any further improvement in a Coriolis-type mass flowmeter employing only one essentially straight Coriolis measuring tube is attainable only by keeping the center of mass of the entirety of the components contained in the compensating cylinder stationary, i.e. the center of mass of the entire assembly encompassing the Coriolis measuring tube, the oscillator or oscillators and the detector or detectors must remain stationary. If there are any other components located within the compensating cylinder, they, too, must, of course, be included in the xe2x80x9cstationary center-of-massxe2x80x9d design.
It is the objective of this invention to further improve on the earlier design of the mass flowmeter employing the Coriolis principle, with regard to the problems detailed above which stem from the use of a single straight Coriolis measuring tube in the mass flowmeter.
The mass flowmeter according to this invention, the design of which solves the problems indicated and meets the stated objective, is basically and essentially characterized in that in terms of mass, the oscillation-capable system consisting of the Coriolis measuring tube and the compensating cylinder is at least to a large extent balanced relative to both the excitation oscillation of the Coriolis measuring tube and the Coriolis oscillation of the Coriolis measuring tube.
In the case of mass flowmeters of the type in question, the Coriolis measuring tube and the components of the oscillator or oscillators and the detector or detectors associated with the Coriolis measuring tube, and, respectively, the compensating cylinder and, where applicable, the components of the oscillator or oscillators and of the detector or detectors associated with the compensating cylinder, are oscillation-capable systems. Yet the assembly consisting of the Coriolis measuring tube, the oscillator or oscillators, the detector or detectors and the compensating cylinder is on its part an oscillation-capable or oscillatory system. The invention is therefore aimed at largely or, in the ideal case, completely equalizing this assembly in terms of mass relative to both the excitation oscillation of the Coriolis measuring tube and the Coriolis oscillation of the Coriolis measuring tube. The term xe2x80x9cequalizingxe2x80x9d in this case means that neither the excitation oscillation nor the Coriolis oscillation is xe2x80x9cactivexe2x80x9d outside the said assembly. The invention is thus based on the determination that, for any further improvement to be attainable in a mass flowmeter operating by the Coriolis principle and employing only one essentially straight Coriolis measuring tube, the center of mass of the said assembly must remain stationary.
Following another, particularly significant underlying concept of this invention, the mass equalization for the excitation oscillation of the Coriolis measuring tube is obtained by means of a first equalizing provision and the mass equalization for the Coriolis oscillation of the Coriolis measuring tube is obtained by means of a second equalizing provision different from the first provision. This can be accomplished in various ways.
To realize the first equalizing provision for the excitation oscillation of the Coriolis measuring tube, a special approach of major significance is characterized in that a first equalizing mass is provided and is connected to the compensating cylinder in the center plane of the latter perpendicular to its longitudinal axis. It is desirable for the first equalizing mass to be so chosen, dimensioned and positioned as to make the natural resonant frequency of the compensating cylinder and the first equalizing mass substantially lower than the resonant frequency of the Coriolis measuring tube.
Particular importance is attributed to a preferred embodiment of the mass flowmeter according to this invention which is characterized in that the connection of the first equalizing mass with the compensating cylinder is rigid relative to the excitation force but soft relative to the Coriolis factor resulting from the Coriolis force. The connection between the first equalizing mass and the compensating cylinder should therefore be implemented in such fashion that the excitation oscillation of the Coriolis measuring tube acts on the first equalizing mass whereas the Coriolis oscillation of the Coriolis measuring tube has no effect, i.e. it cannot act, on the first equalizing mass.
To realize the second equalizing provision for the Coriolis oscillation of the Coriolis measuring tube, a special approach of major significance is characterized in that a second equalizing mass and a third equalizing mass are provided and that the said second equalizing mass and the said third equalizing mass are in the form of end sections of the compensating cylinder, meaning that at its two ends, the compensating cylinder transitions into the second equalizing mass and the third equalizing mass, respectively.
So far, the mass flowmeter design addressed and the mass flowmeter according to this invention have only been described in reference to their significant functional components. A mass flowmeter of the type in question usually also includes an external housing. In that case, the mass flowmeter according to this invention is also preferably characterized in that the assembly composed of the Coriolis measuring tube and the compensating cylinder is supported in and attached to the housing by way of the ends of the Coriolis measuring tube or via special connecting tubes.
If and when the mass flowmeter according to this invention also includes the usual external housing, the structural design and geometric shape of the housing is important for optimal performance of the mass flowmeter according to this invention.
In typical fashion the external housing of the mass flowmeter according to this invention consists preferably of a circular, cylindrical casing and two housing end caps. It is desirable to configure the latter in a special way, characterized in that each housing end cap includes an end plate which is at least approximately cylindrical and faces away from the casing, and a connecting element which faces the casing.
In terms of the design of the external housing, i.e. the housing end caps thereof, a particularly preferred embodiment of the mass flowmeter according to this invention is characterized in that the outer diameter of the cylindrical end plates of the housing end caps is limited to what is functionally necessary and that, in the longitudinal direction, the connecting elements have an at least approximately parallelogram-shaped cross section, with the longer axis of the parallelogram extending from the end plate to the casing while the cross-sectional area of the connecting elements is substantially larger than the cross-sectional area of the end plates.